This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-146529 filed May 18, 2000.
The present invention relates to a technology for calculating an average vehicle speed.
A navigation system provides an appropriate travel route from a present position to a destination for use as a travel guidance, for instance, time of arrival at the destination or a passing point is estimated and notified for user""s convenience. In estimating the arrival time, required time is calculated by dividing a distance from the present position to the destination or the passing point by an average vehicle speed.
For determining this average vehicle speed, U.S. Pat. No. 5,928,308 (JP-A-9-96539), for instance, teaches to set an average vehicle speed for each type of roads based on individual sense of users (drivers). In this case, the average vehicle speed deviates from an actual vehicle speed, because it is determined based on user""s individual sense and fixed unless changed by the user.
It is also proposed to determine an average vehicle speed automatically from a distance actually traveled and time required to travel the distance (for instance, JP-A-8-22247). In this instance, the above deviation between the speed set with the driver""s sense and the actual speed is eliminated. However, the average vehicle speed is calculated based on changes in distance from the present position to the destination irrespective of the types of roads. For instance, vehicle speeds are averaged even if the average vehicle speeds differ tens of kilometers per hour between a general road and an expressway. As a result, the average vehicle speed cannot be calculated accurately after the road type is changed when the type of road changes. Thus, the average vehicle speed cannot be calculated accurately if the required time is calculated by using the above vehicle speed.
Therefore, the present invention has an object to provide a technology for automatically calculating an average vehicle speed accurately.
According to the present invention, a road type determination unit determines a type of road a vehicle is traveling at present based on a present position of a vehicle. A travel distance calculation unit calculates a travel distance of the vehicle based on a change in the vehicle position. An instant vehicle speed calculation unit calculates, when the travel distance reaches a predetermined determination reference under a condition that the road type is the same, an instant vehicle speed of the vehicle for each road type based on the predetermined determination reference and time (required travel time) required to travel the predetermined determination reference. This may be calculated, for instance, by dividing the determination reference by the required travel time. An average vehicle speed calculation unit calculates an average vehicle speed for each road type by using the thus calculated instant vehicle speed.
Preferably, the required travel time used for calculation of the instant vehicle speed excludes time of rest of the vehicle. It is preferred that the determination reference is set variably, and a rate of use of the instant vehicle speed in a calculation of the average vehicle speed is set variably.